Optical stress concentration and stress gradient monitoring during elasto-plastic fatigue tests with Digital Image Correlation

Abstract

Predictions of fatigue failure at stress concentrations rely on prediction by empirical formulae or experimental test data. However, there remains a level of uncertainty in the life estimation, which can be reduced when monitoring the behavior around the stress concentration. In this contribution, the stress field, calculated for an isotropic material from optical strain measurement, is monitored over specimen fatigue life. Changes in the maximum relative stress gradient χmax, describing the decay of a stress concentration, and the distance of the maximum gradient from the hole are evaluated. Experimental data on three specimens with different load levels in the high-cycle fatigue range show that both parameters change significantly shortly before fatigue failure. These encouraging results indicate that failure can be predicted by monitoring either, the value of χmax, or the distance of χmax from the hole edge.